Mineral binders such as concrete, cement, mortar, and exterior plaster are often colored to enhance their aesthetic appeal. Coloring can be accomplished either by applying a colored coating to exposed surfaces of a cured or set mineral binder or by premixing small amounts of one or more pigments in a mineral binder system prior to setting to color the system uniformly. The pigment(s) in the premix method can be added either to a dry mineral mix, as for example, in the case of concrete, to the cement-sand mixture, or can be added to water used to set the mineral mix. The premixing method of coloring is preferred because colored surface coatings are not permanent but peel, fade, and hence weather more rapidly than premixed colorants.
Pigments for mineral binder systems which are exposed to outdoor conditions must be alkali-resistant, lightfast, resistant to industrial atmospheres, and must weather at a rate comparable to the rate at which the binder itself weathers so that surface appearance does not change unevenly over time. Additionally for ease of handling, the pigment should be relatively dust-free to facilitate mixing to achieve maximum coloring power and should be easily dispersible.
Black pigments are particularly desirable colorants for mineral binder systems because a large variety of colors and color shades can be obtained by their use either alone or in combination with other pigments. Black iron oxides and carbon blacks are common black pigments.
Carbon blacks exhibit excellent coloring properties, alkali-resistance, lightfastness, and chemical stability. However, carbon blacks are not preferred pigments in mineral binder systems exposed to outdoor weathering because the surface appearance of bodies containing carbon black exhibit undesirable changes such as fading with weathering. Fading is attributed to a preferential leaching out and washing away of the carbon black pigment from the mineral binder system. Furthermore, when carbon black is used in combination with other colorants, the appearance of the other colorant becomes more pronounced with weathering over time due to the selective leaching out and washing away of carbon black pigment. Consequently, carbon black has had limited use in systems which are exposed to outdoor weathering.
Another drawback to the use of carbon black as a colorant in indoor or outdoor applications is that, depending on the physical form in which it is supplied, carbon blacks are either excessively dusty or difficult to disperse. The process used for incorporating carbon blacks into binder systems depends on the form in which the pigment is supplied and on the processing equipment available to the user. Powdery carbon blacks have bulk densities ranging from about 0.02 to 0.1 g/cc and are termed "fluffy" blacks. Because of their low densities and large surface areas, the fluffy products are cohesive, have very poor conveying properties, are difficult to handle in bulk, and are usually supplied in bagged form. However, fluffy carbon blacks are dispersible and can develop full coloring potential by relatively simple grinding procedures.
Handling properties for a given grade of carbon black improve with increased densification such as by pelletization. For example, fluffy blacks are typically densified by pelletizing to improve bulk handling properties. Such procedures provide bulk densities ranging from about 0.2 to 0.7 g/cc. However, dispersibility is progressively degraded as the pellet density increases. Thus, there is a tradeoff between improvement in bulk handling properties and degradation in dispersibility. Because of the advantages of increased cleanliness, pelletized carbon blacks are generally preferred for introducing carbon blacks into mineral binder systems despite the need for increased grinding or milling to form a uniform, intimate mix.
Carbon black agglomerates, whether in pelletized or fluffy form, must be broken down primarily to aggregates (smallest dispersible units of carbon black) for full color development. This is accomplished either by milling the dry mix or by predispersing the carbon black in the aqueous medium. For example, in the preparation of colored concrete, carbon black can be milled into a dry sand-cement mixture, and subsequently, the requisite amount of water necessary for setting the mix can be added. Alternatively, an aqueous dispersion of the carbon black, in all or part of the requisite volume of water necessary for setting the mix, can be uniformly blended into the sand-cement mixture.
Since carbon blacks tend to be hydrophobic, surface active agents are often used to promote wetting, to enhance the dispersion process, and to aid dispersion stabilization. For example, U.S. Pat. No. 4,006,031 discloses the use of fluorine-containing wetting agents used with carbon blacks in an attempt to improve the weathering properties of mineral binder systems. European Patent Publication No. 50354 describes the use of surface-active polymers which disperse the carbon black in an aqueous medium used to set the mineral binder system. The polymer becomes inactive after drying. However, in dry milling or predispersion, the carbon black, whether in fluffy or pelletized form, must still be milled to attain satisfactory pigment dispersion.
Thus, there remains a need for carbon black colorants for mineral binder systems which exhibit both improved weatherability properties and/or are easily dispersed in pelletized form.